The overall objective of this study is to characterize and define physiological and biochemical changes which may occur in the hearts of pigs after an endurance exercise training program. Studies to date do not support the concept that endurance exercise increases the arterial collaterals in the hearts of exercise trained pigs. Collateral blood flow was the same in trained versus non-trained animals. Preliminary biochemical analyses suggest that some alterations may occur in the enzyme activity of heart muscle after a period of training. Succinate dehydrogenase and cytochrome -C oxidase in both crude homogenates and mitochondrial preparations show greater activity in exercise trained versus sedentary control animals. However, more studies are needed to clarify these relationships. The present goals are to continue physiologic, biochemical and morphologic studies of heart muscle in exercise trained miniature pigs in order to assess the effect which training may bring about.